Oxidation and reduction reactions are of great use in a variety of areas including for the production of energy, removal and/or conversion of unwanted compounds or chemicals, and the generation of desirable products. These reactions typically use catalysts to enhance a chemical reaction either electrochemically, wherein electrons are transferred from one chemical species to another or homogeneous/heterogeneously, where chemical reactions are facilitated by the active sites of a catalyst. Oxidation is the result of a loss of electrons or an increase in oxidation state while reduction is the result of a gain in electrons or a decrease in the oxidation state.
Catalysis can be either homogenous or heterogeneous. Homogenous catalysis is where the catalytic reactions take place in the same phase as the reactants. Most typically, homogenous catalysis takes place in solution and thus requires the chemical or compound undergoing the reaction to be in solution with a soluble catalyst. In contrast, heterogeneous catalysis is when catalysis takes place in the interface of two phases, typically: gas-solid, gas-solvent, solvent-solid etc, and thus allows for the chemical or compound undergoing the reaction to be in the gas (solution) phase while the catalyst is typically solid.
A number of desirable oxidation and reduction reactions have thus far only been shown to be catalyzed by homogenous catalysis. Examples include carbon monoxide oxidation, carbon dioxide conversion reactions and oxyfuel oxidation/reduction reactions.
Carbon dioxide (CO2) is generated in substantial quantities by carbon based energy generating processes such as burning methane (and other hydrocarbons) or coal which account for 82% of the worldwide energy supply in 2014. The accumulation of atmospheric CO2 can be attributed to the unregulated and continuous rate of its release since the industrial revolution, resulting in an increase in concentration from 278 ppm to 400 ppm. The current concentration of CO2 strains the balance of the earth's natural carbon cycle, resulting in acidified oceans and irregular weather patterns. Climate models predict drastic changes in precipitation and temperature, which will impact regional and global economies. A general consensus that CO2 emissions will continue requires the development of technologies that will result in ambitious emission reductions over the next few decades. Current solutions to decrease CO2 emissions include increasing the percentage of energy generated from renewable or nuclear sources and utilizing CO2 capture and storage (CCS) at point sources (e.g. power plants) which can be extended to CO2 capture and conversion (CCC) into fuels or value added products. See, e.g., UNEP 2013. The Emission Gap Report 2013. United Nations Environmental Programme (UNEP), Nairobi; Sanchez-Sanchez C M et al., Electrochemical approaches to alleviation of the problem of carbon dioxide accumulation. Pure Appl. Chem. 2001; 73:1917-1927; and Lim R J, et al., A review on the electrochemical reduction of CO2 in fuel cells, metal electrodes and molecular catalyst. Catal. Today. 2014; 233:169-180. CCC would transform carbon emitting energy sources into carbon neutral processes by reintroducing CO2 into the energy cycle, a process that can be implemented in batch or flow reactors as shown in the selective reduction of CO2 to formic acid in optimized electrochemical systems with high conversion rates on Pb (94.1%) and Sn (70-80%) (See, e.g., Lowry D, et al., Electroreduction of Carbon Dioxide in Electrochemically Enabled Sustainability: devices, materials, and mechanisms for energy conversion; Chan K Y, Li C Y. V. Eds. Taylor & Francis Group, LLC, 2014, 1-54) and the “one-pot” synthesis of controlled molar ratios of H2 and CO (syn-gas) on Ag (See, e.g., Delacourt C, Newman J. Mathematical modeling of CO2 reduction to CO in aqueous electrolytes II. Study of an electrolysis cell making syngas (CO+H2) from CO2 and H2O at room temperature. J. Electrochem. Soc. 2010; 157:B1911-B1926. Delacourt C, et al., Design of an Electrochemical Cell Making Syngas (CO+H2) from CO2 and H2O Reduction at Room Temperature. J. Electrochem. Soc. 2008; 155:B42-B49; and Dufek E J, et. al., Bench-scale electrochemical system for generation of CO and syn-gas. J. Appl. Electrochem. 2011; 41:623-631). Further, CCC could be integrated with intermittent renewable energy sources (e.g., solar or wind) to store energy as reduced CO2 in liquid or gas products for later use.
Similarly, the importance of oxalic acid, especially for clinical diagnosis and in food and water technology, is now widely recognized. Small carboxylic or dicarboxylic acids (such as formic, maleic, and acetic acids) are intermediate products in catalytic oxidation of aromatic compounds present in wastewater. At the same time oxalic acid is a compound that is toxic to almost all organisms. The accumulation of oxalic acid causes hyperoxaluria, formation of calcium oxalate stones in the kidney, renal failure, cardiomyopathy, and cardiac conductance disorders.
Electrochemical oxidation of organic compounds is a widely used approach for wastewater purification, energy production, and synthesis of value added products. Metal oxides, like copper, zinc, manganese and supported noble metals, like platinum group metals (PGM) or zinc, are used as catalysts in these processes due to their high activity and lack of selectivity. Platinum and ruthenium carbon supported catalysts are shown to be efficient for the oxidation of different carboxylic acids. For example, oxalic acid can be oxidized using platinum in mild experimental conditions and pH=0 at potential values between 0.7 and 1.8 V vs. RHE with a maximum catalytic activity at 1.3 V. Unfortunately use of noble metal catalysts such as platinum group metals is expensive, which hinders their large-scale application for wastewater purification. In addition, in metal-catalyzed oxidation there is a risk of irreversible deactivation due to the modifications of the metal surface in the course of reactions by metal sintering or poisoning of the surface by strongly adsorbed species such as oxygen. Metal leaching away in the solution is another known problem with the use of composite metal oxides.
The ability to engage in electroreduction, electrooxidation or electrochemical conversion of compounds such as carbon dioxide, carbon monoxide, and hydrogen electrooxidation or evolution (HOR and HER, respectively) and oxidation/reduction of oxyfuels in, for example, the gas phase via the use of heterogeneous catalysts (or electrocatalysts) is highly desirable as this pathway would open up significant avenues for processing of these compounds in phases in which they are commonly found.